1. Field of the Invention
This invention relates to an image forming method and an image forming apparatus which make use of a two-component developer having a toner and a carrier, used in electrophotography or electrostatic recording and so forth, to develop an electrostatic latent image on a photosensitive member to form an image.
2. Related Background Art
In general, in image forming apparatus by which images are recorded on recording mediums such as paper, as in printers, an electrophotographic system is employed as a system by which the images are recorded on the recording mediums.
The electrophotographic system employs as an electrostatic latent image bearing member a photosensitive drum coated with a photosensitive material on its surface. First, the surface of the photosensitive drum is uniformly electrostatically charged, and thereafter the surface of the photosensitive drum is exposed to laser light, so that a potential difference is given between exposed areas and unexposed areas.
Next, using a developer having a toner and a carrier, a toner standing charged is made to adhere to the surface of the photosensitive drum in accordance with the potential difference to form a toner image on the surface of the photosensitive drum. Thereafter, the toner image is transferred to a recording medium, and then the toner image is fixed onto the recording medium by the action of heat and pressure or the like to form an image.
Recently, as reproduction equipment has attained higher image quality and higher speed, toners and photosensitive drums have been strongly required to achieve low running cost and so forth. As photosensitive drums used in the electrophotographic system, those having photosensitive layers smaller in thickness are used because of a necessity for high resolution. In addition thereto, it is attempted to improve electrical and mechanical strength or wear resistance of photosensitive drum surface because of a necessity for making the photosensitive drums have a longer lifetime for the achievement of low running cost.
As disclosed in Japanese Patent Application Laid-open Nos. H05-216249 and H07-072640, a proposal to use a curable resin in the surface layer is made in order to make the photosensitive member have higher resolution and higher durability. Where such a curable resin is used in the surface layer, compared with a thermoplastic resin, the surface layer can have a higher mechanical strength not to be easily abraded and also not to be easily scratched, and hence can have a longer lifetime. As also disclosed in Japanese Patent Application Laid-open No. 2003-345049, a method is proposed in which, in order to make the photosensitive member have higher durability inclusive of higher resolution as well, a surface layer that utilizes a variety of energy (such as heat, ultraviolet rays and electron rays) is formed on the surface of the photosensitive member (see Example 1). This brings about superior potential characteristics and image characteristics at the initial stage and also after running (comprehensive operation) even though thin films are formed which are 15 μm in layer thickness as the total of a photosensitive layer and the surface layer.
In such a thin-film photosensitive member, compared with thick-film photosensitive members, latent images which are sharp and have a high electric-charge density are formed when the same images are tried to be formed. Where such latent images are developed, sharper images are formed, desirably. However, in characteristics of photosensitive member potential and image density, the electric-charge density comes higher in inverse proportion to the layer thickness of the photosensitive member, and hence, in order to fill such electric charges with the toner, the toner becomes larger in quantity in inverse proportion to the layer thickness of the photosensitive member. That is, the γ curve may inevitably have a sharp slant (for example, if the layer thickness of the photosensitive member is half, the electric-charge density that is necessary to obtain a certain potential doubles. In order to fill such electric charges with the toner, the toner doubles in its quantity. In other words, the image density may abruptly vary with respect to the variations of contrast potential). Where such a photosensitive member is used, very clear images are obtainable in black-and-white copying machines or printers. However, in full-color copying machines or printers, which are required to provide high image quality for halftone, problems may arise such that it is difficult to reproduce the halftone because the γ curve has a sharp slant in the halftone region, and that image density varies greatly because of a slight variation in potential to make it difficult to reproduce colors. Also, in the thin-film photosensitive member, the potential of contrast tends to be so small that the above tendency is apt to be more remarkable.
Meanwhile, as disclosed in Japanese Patent Application Laid-open No. H06-332237, an approach is made from developers in order to achieve a higher image quality. What is proposed therein is that, for the achievement of a higher image quality, a toner is made to have small particle diameter and its particle size distribution is specified and further a carrier is made to have small particle diameter and its shape is specified, to achieve superior fluidity and superior resolution, gradation and fine-line reproducibility. Making toners and carriers have smaller particle diameter is very effective in making image quality higher. However, in the case when the thin-film photosensitive member as stated above is used, which has a high resolution but has a sharp slant in the γ curve, it is necessary for the toner to have a very large charge quantity in order to make the gradation high, and developing performance, i.e., what is called “toner release” from carrier, is not sufficiently satisfied in some cases. Also, if the toner has a large charge quantity, counter charges are liable to be collected in the carrier to cause carrier adhesion. On such an occasion, if the carrier has an amorphous particle shape, the photosensitive drum tends to be finely scratched at its surface even when it has a tough protective layer, so that the surface of the photosensitive drum may become matte with progress of running to cause coarse images or lines at halftone areas.